This specification relates to modifying glyph outlines.
A character is an abstract construct that often represents an atomic unit in some system of expression, e.g., a language. Each character can be represented by a set of character attributes that define the semantic information of the character. A character encoding associates the set of character attributes for a character with a particular encoding value—for example, a scalar value included in a character set standard, such as ASCII (American Standard Code for Information Interchanges) or Unicode.
A glyph is a visual representation of a character, such as a graphical token or symbol, and includes one or more strokes, which may be vertical or horizontal (sometimes referred to as vertical and horizontal stems), curved or angled. A glyph image is a particular image of a glyph that has been rasterized or otherwise imaged onto some display surface. A font is a collection of glyphs, and can include a mapping of the collection of glyphs to corresponding characters (i.e., to encoding values). A font is typically constructed to support a character set standard. That is, fonts include glyphs representing characters included in the character set standard. A glyph can be associated with a set of glyph attributes defining appearance information for a representation of the corresponding character, and the glyph provides the information necessary to render a corresponding glyph image. A glyph can include, or can be associated with, a set of instructions for rendering the glyph. For example, TrueType™ fonts, available from Microsoft Corporation of Redmond, Wash., include glyphs that are associated with a set of instructions for use when rendering the glyph.
Hinting is a method of defining which pixels are turned on in order to create the best possible glyph bitmap shape, particularly at small sizes and low resolutions. A glyph's outline determines which pixels will constitute the bitmap. It is often necessary to modify the outline to create the bitmap, i.e., modify the outline until the desired combination of pixels is turned on. The modified outline can be referred to as a hinted outline. In certain fonts, such as TrueType™ fonts, a hint is a mathematical instruction that is included in the font program that defines a distortion of a glyph's outline at particular sizes. In other fonts, such as Type 1 or compact font format (CFF) fonts available from Adobe Systems Incorporated (“Adobe”) of San Jose, Calif., a glyph outline may be hinted according to various hinting policies. Hinting of Type 1 fonts is described in Section 5 of a manual entitled “Adobe Type 1 Format”, Version 1.1 available from Adobe. The term “Type 1” as used herein includes font/glyph definitions that are derived from or an enhancement of the Adobe Type 1 font format.
Before digital typography and scalable type, a font typically had a unique design for each glyph at each size. While the designs at different sizes were similar, important differences existed. As the size of a glyph became smaller, the relative size of the stems increased and the relative spacing between glyphs increased. These differences can be collectively referred to as optical compensation. However, with the advent of digital typography and the implications of “what you see is what you get” (WYSIWYG) and linear scaling, the optical size refinement in type design was largely lost. There are exceptions. For example, MultipleMaster fonts available from Adobe may have an optical size axis, or an individual type design can be implemented for different design sizes. However, even these techniques do not work well for final form documents that may be displayed at different zoom levels.